This invention generally relates to a pulley and a ball bearing for a pulley. In particular, the invention relates to a ball bearing for use in a timing belt of an engine of an automobile, and a pulley to be contacted with a belt for driving accessory of an automobile.
In a timing belt for an engine of an automobile, or an accessory driving belt, a pulley is arranged so as to increase an angle of contact of a belt for the purpose of applying an appropriate tension to the belt. FIG. 12 shows a constructional example of a timing belt. This timing belt includes a belt E which is stretched around a driving pulley A secured to a crankshaft, driven pulleys B, B secured to a camshaft, a tensioner pulley C rotatably mounted on a stationary shaft, and an idler pulley D mounted on a stationary shaft. Among the above-mentioned pulleys, the pulleys C and D mounted on the respective stationary shafts are usually constituted by a ball bearing supported pulley. The ball bearing is fitted within the inner diameter of the pulley. It is noted that the stationary shaft for the tensioner pulley C is displaceably supported in elongated openings provided in a frame for supporting the tensioner pulley. By this, tension adjustment of the belt is permitted.
Such a kind of pulley may include a pulley in which a peripheral surface of the pulley to be engaged by a belt is provided directly around the outer diameter of an outer ring of a ball bearing (called a hat type outer ring). It is noted, however, that a pulley of the type shown in FIG. 13 is frequently used. This pulley includes a pulley body 11 having a peripheral surface 11c and a ball bearing 12 which are engaged with each other as a unit.
The pulley body 11 is pressed from a steel plate. The pulley body 11 includes an outer cylindrical portion 11a around which a belt is extended, and an inner cylindrical portion 11b fitted over an outer ring 12a of the ball bearing 12. A peripheral surface 11c of the pulley is provided around the outer diameter of the outer cylindrical portion 11a. The ball bearing 12 includes the outer ring 12a fitted within the inner cylindrical portion 11b of the pulley body 11, an inner ring 12b fitted over a stationary shaft (not shown), a retainer 12d for retaining a plurality of balls 12c provided in a space between the raceway surfaces of the inner and outer rings 12b and 12a, and a seal 12e for sealing a grease.
In the above-mentioned pulley, and when the pulley body 11 is rotated by a rotational driving force from the belt, the outer ring 12a of the ball bearing 12a fitted within the pulley body is rotated integrally with the pulley body 11.
In the above-mentioned pulley, the ball bearing 12 is of a outer ring rotation type, and usually, a deep groove ball bearing is used provided with zero contact angle.
When the above-mentioned pulley is driven under cold ambient, abnormal noise (hoot sound) may be caused, depending upon a given specification of the pulley and/or operational condition. Such abnormal sound under cold condition, i. e., abnormal noise under cold ambient is not always caused during use. Occurrence of such abnormal noise under cold ambient greatly depends upon a given ambient temperature. Thus, it occurs only in a very limited area (for example, Hokkaido Prefecture in Japan). Such abnormal noise under cold ambient occurs in a brief time (within one minute at the longest) after an engine start of an automobile. Thereafter, no abnormal noise is caused. Abnormal noise is frequently caused when a deep groove ball bearing of a single row type is used as a ball bearing. Occurrence of abnormal noise is reduced when an angular ball bearing of a plural row type is used.
Abnormal noise under cold ambient has a complexed nature as mentioned above, so that it is difficult to reproduce such abnormal noise under cold ambient. Accordingly, the cause or mechanism for the occurrence of abnormal noise has not been definitely clarified. As far as a pulley for use with an automobile is concerned, it is operated at a high temperature and at a high speed. Thus, durability is one of the important factors for such pulleys. Accordingly, any counter-measures leading to a reduction in durability could not be employed.
For the reasons given above, no particular means for elimination of such abnormal noise under cold ambient has been proposed heretofore.
Conventionally, a grease having a superior property at lower temperature (being able to form uniform oil film in the contact portions between the rolling element and the raceway surfaces of the inner and outer rings) has been proposed in order to eliminate abnormal noise under cold ambient. This counter-measure is intended to restrict occurrence of abnormal noise under cold ambient by increasing lubrication ability of a grease under cold ambient, and expected to give a relatively high effect. It is noted, however, that such a grease has an lower viscosity, so as to provide an insufficient lubrication ability at high temperature. This may lead to a reduced durability.
Accordingly, it is the main object of the invention to provide means, keeping the function of a pulley, for effectively restricting or preventing occurrence of abnormal noise under cold ambient, while, at the same time, satisfying durability and cost effectiveness.